1. Field of the Invention
The present invention relates to correcting variations in the intensity of light within an illumination field without distorting the telecentricity of the light.
2. Related Art
In an integrated circuit chip, devices and their connections are typically fabricated in phases. Several phases include processes that modify portions of a semiconductor substrate (i.e., a wafer). For each of these phases, the portions to be processed must be isolated from the remaining portions of the wafer. Typically this is accomplished by replicating a two dimensional pattern on a surface of the wafer. Photolithographic tools that effect this replication are often referred to as wafer steppers or wafer scanners.
The pattern distinguishes the portions of the wafer to be processed from the remaining portions. The pattern is formed on a mask (i.e., a reticle) having corresponding opaque and either transparent or reflective portions. A beam of light is made to impinge upon the reticle so that the pattern is transferred to the cross section of the beam perpendicular to its direction of propagation (i.e., the illumination field). (The illumination field usually is shaped as a rectangle or an annulus sector.) Optical devices positioned along the optical path between the reticle and the wafer reduce the size of the pattern and cause the beam of light to impinge upon the wafer. A layer of film (i.e., a photoresist) is applied on the surface of the wafer. When the photoresist is exposed to the beam of light, corresponding portions of the photoresist are exposed. Either the exposed or unexposed (but not both) portions of the photoresist are removed to reveal the underlying portions of the wafer to be processed. The remaining portions of the wafer are protected from the process by the remaining photoresist.
Undesirable variations in positions or feature sizes can affect the electrical and electronic characteristics of the fabricated devices. Therefore, it is important to be able to control positions and feature sizes precisely. When the wavelength of the light that impinges upon a surface of a wafer is on an order of tens to hundreds of nanometers, variations in the flatness of the wafer surface can be significantly large enough to preclude modeling the wafer surface as a single image plane. For this reason, it is important that the light that impinges upon the wafer surface be substantially telecentric.
Moreover, variations in the intensity of the light at points within the illumination field can affect the degrees to which the photoresist at these points are exposed. For this reason, it is also important to correct for variations in the intensity of the light within the illumination field. Therefore, what is needed is an apparatus and method for correcting variations in the intensity of the light within the illumination field without distorting the telecentricity of the light that impinges upon the layer of photoresist.